Stop system select from croaking on read-only COW ""

Stop system select from croaking on read-only COW ""

System select (select with 4 arguments) does not allow any of its
first three arguments to be read-only unless they are undef or
empty strings.

It does not work properly for read-only copy-on-write empty strings,
because it passes all copy-on-write through sv_force_normal under the
expectation that they will shortly be modified.  It should not be
doing that for read-only empty strings.